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Pixels: Background, Explanation, and Dynamics

Updated on November 22, 2014

The Question

I recently had a conversation with a friend about digital image structure in which I had to explain canvas size and general image layout... Getting into the nitty gritty, I got the question, "what are pixels?" I have to admit that it's a very valid question and, unfortunately, one that often isn't answered properly (in my humble opinion).

If you want a technical description of pixels, their origins, and their functions in a multitude of applications, have a look at this Wikipedia article. Otherwise, you can read the "plain English" explanation here as it pertains to pixels in digital imaging and printing.

Digital Imaging

In a digital image, a pixel is a physical component, the smallest controllable element on your screen. You're looking at pixels now, reading them... after all, text is just a structured layout of pixels to create an image for you to interpret.

Even with no picture - just a blank canvas - pixels exist. Each pixel has a physical coordinate on the screen and can also be used as a form of measurement for image height and width. The display size of pixels on your screen depends on screen resolution (and we'll briefly discuss that later).

Portion of image enlarged to show individual pixels rendered as small squares.
Portion of image enlarged to show individual pixels rendered as small squares. | Source

Pixels are often referred to as dots, but dots are round. It may be more appropriate to think of pixels as squares. As a collection of squares, pixels make up an image on your screen. More pixels in a small physical area create higher resolution and generally provide a more defined, crisper image.

Pixels are normally arranged in a two dimensional grid (for lack of a better term) where each square on the grid can fit one (and only one) pixel. This is where we get into the "size" of the grid and, subsequently, screen resolution...

Enlarged example of a normal grid with some individual pixels in red, green, and blue.
Enlarged example of a normal grid with some individual pixels in red, green, and blue.

Screen Resolution

The display resolution of a digital device (like your smartphone, computer monitor, or a television) is measured as the number of individual pixels that can be displayed in each dimension. We're working with two dimensions here...

The resolution is generally quoted as pixel width multiplied by pixel height. There are an awful lot of possibilities, so I won't list all of them here. Some notable "common" resolutions include 640x480, 1024x768, 1920x1080, etc.

Let's take 1024x768, as an example. A screen with a display resolution set to those values would accommodate an image that is 1024 pixels wide and 768 pixels tall (or anything smaller, obviously).

In terms of image resolution, as on your screen, pixel density is often referred to as ppi - pixels per inch.

Print Resolution

In printing, pixels are also important. However, rather than being called pixels on paper, we think of them as ink droplets. Generally, the more pixels a printer can produce within a given area, the higher the resolution of the image. Higher resolution - or pixel density - creates a better representation of the original digital image. In other words, (much like with digital image resolution on your screen) greater pixel density produces a clearer, crisper image.

Printer resolution is commonly referred to as dpi - dots per inch.

300dpi on the left, 72dpi on  the  right.
300dpi on the left, 72dpi on the right.

How Big Is a Pixel?

As I mentioned in the beginning, the display size of pixels as you see them on your screen or on paper (with a printed image) is dependent on resolution. This is where ppi and dpi come in. Remember pixels per inch and dots per inch?

The measurement is exactly what the name implies... with ppi, for example, the measurement is the actual amount of pixels per square inch. Higher ppi means greater pixel density which, in turn, means higher image resolution.

The same is true with dpi in printing. Higher dpi means that there are more ink dots per square inch, therefore the image is printed in higher resolution.

The Pythagorean theorem, illustrated.
The Pythagorean theorem, illustrated.

Calculating Pixel Density

You want to calculate pixel density? Really? Alright, alright... We'll need some measurements.

Let's make the assumption that the image on the right represents your display screen (your computer monitor) and that it's a 22 inch screen, as advertised by the manufacturer. As a note, the monitor size is generally advertised as a diagonal measurement of the display.

So, "a" is the display height in pixels, "b" is the display width in pixels, and "c" is the display's diagonal (again, in pixels). We generally know "a" and "b" simply by looking at the display resolution setting. The "c" value, on the other hand, must be calculated.

If we take the example in the screen resolution section above, the display resolution is set to 1024x768 (which is rather low for a 22 inch monitor). We can apply the Pythagorean theorem to calculate "c" as 1280 pixels. But don't stop there... To calculate pixels per inch, divide "c" by the declared monitor size (the 22 inches for this example) and we get a ppi value of 58.18. Not very impressive.

But, bump that resolution to... say... 1920x1080 and we get a "c" value of about 2203 pixels. Divide that by your 22 inch screen measurement to get 100.14 pixels per inch. Now we're getting somewhere. :)

As a note, these calculations won't be particularly precise unless we know the exact measurement (in inches) of the display area. Manufacturers don't have a universal guideline for labeling display diagonals, so some may label slightly differently than others. It's always best to check it yourself - measure just the display area of your monitor, not the frame.

Pixelated Summary

So there we have it... Pixels. Pixels everywhere.

  • Pixels are used in digital imaging to refer to the smallest controllable element of an image.
  • Screen resolution is measured in pixel width and height.
  • Digital image resolution is described in ppi - pixels per inch.
  • Printed resolution is a measurement of ink dot density on paper and is described in dpi - dots per inch.
  • Pixel display size is directly dependent on display resolution.
  • Pixel density can be calculated with a simple application of the Pythagorean theorem.

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